1. Technical Field
For example, the present invention relates to a transmitter using an analog quadrature modulator. More particularly, the invention relates to a technique for satisfactorily compensating for a distortion occurring in a transmission signal by detecting a linear distortion before and behind a signal processing section which is or includes an analog quadrature modulator and performing a control so that the difference between the detected distortions, which is regarded as occurring in the signal processing section, is made small (e.g., minimized).
For example, the invention relates to the following techniques.
The invention relates to a transmitter and a carrier leak detection method. More particularly, the invention relates to a transmitter and a carrier leak detection method for radio-transmitting a high-precision quadrature modulated wave by detecting and compensating for a carrier leak occurring when an analog quadrature modulator is used (e.g., first to third embodiments).
The invention also relates to a transmitter or the like using an analog quadrature modulator which radio-transmits a high-precision quadrature modulated wave by compensating for a distortion occurring due to an I-phase/Q-phase gain imbalance occurring between two D/A converters that are provided separately for the I phase and the Q phase (e.g., fourth embodiment).
The invention also relates to a transmitter or the like using an analog quadrature modulator which radio-transmits a high-precision quadrature modulated wave by detecting and compensating for a deviation in orthogonality (e.g., fifth embodiment).
Furthermore, the invention relates to a transmitter or the like which detects and compensates for two or more of errors as mentioned above, that is, a carrier leak (DC offset), an I/Q gain imbalance, and a deviation in orthogonality (e.g., sixth embodiment).
2. Description of the Related Art
For example, in the transmitter of a wireless communications apparatus which performs a wireless communication using W-CDMA (wide-band code division multiple access) or the like, a signal as a transmission subject is quadrature-modulated by using an analog quadrature modulator. However, a distortion occurs in a transmission signal due to a carrier leak, an I/Q gain imbalance, or a deviation in orthogonality. It is necessary to compensate for such a distortion.
This will be described below in more detail.
In the following, “t” represents time and may be information of time itself, a sampling number, or the like.
First, the carrier leak will be described.
In a transmitter used in a mobile communications system of W-CDMA or the like, generation of a carrier leak is unavoidable as long as a signal is processed by using device for processing an analog signal such as a D/A converter or an analog quadrature modulator. D/A converters should output an analog signal of 0 V in response to input digital data “0.” However, an adjustment for that purpose is difficult. Even if an adjustment is made, a deviation tends to occur again due to a temperature variation or a variation with age. As a result, an analog DC signal whose level has a non-0-V offset is output in response to digital data “0.” This DC component is up-converted to an RF band by the analog quadrature converter and becomes a carrier leak. Furthermore, in the analog quadrature converter, part of a local signal that is input to it leaks so as to be included in a transmission signal, which means generation of a carrier leak. A carrier leak should be compensated for to satisfy a spurious standard. Even if a carrier leak is within a modulated wave band, it is a factor in deteriorating the signal quality. Therefore, a carrier leak should be compensated for correctly even during operation (refer to Patent documents 1-8, for example).
FIG. 1 shows the configuration of a conventional transmitter with carrier leak compensation.
A digital modulating means 1 performs bandwidth limitation and digital quadrature modulation to respective carrier frequencies on input baseband signals and combines resulting signals. The digital modulating means 1 thus outputs a first IF (intermediate frequency) multicarrier signal.
A carrier leak compensating means 2 adds carrier leak compensation values CLCancelI and CLCancelQ which are opposite in phase to carrier leaks to the output of the digital modulating means 1 according to the following equations:
Formulae 1:TxI′(t)=TxI(t)+CLCancelI TxQ′(t)=TxQ(t)+CLCancelQ  <Formulae 1>where
TxI(t): I-phase signal of the output of the digital modulating means;
TxQ(t): Q-phase signal of the output of the digital modulating means;
TxI′(t): I-phase signal of the output of the carrier leak compensating means;
TxQ′(t): Q-phase signal of the output of the carrier leak compensating means;
CLCancelI: I-phase carrier leak compensation value; and
CLCancelQ: Q-phase carrier leak compensation value.
A D/A converter 3 converts the transmission digital signal (I component and Q component) received from the carrier leak compensating means 2 into an analog signal.
An analog quadrature modulator 4 converts the complex signal received from the D/A converter 3 into a real signal, up-converts it to a desired RF (radio frequency) band, and outputs a resulting signal. The transmitter with carrier leak compensation transmits this RF signal.
A frequency converting means 5 down-converts a signal that is based on the output of the analog quadrature modulator 4 to a second IF.
An A/D converter 6 samples and quantizes an output signal of the frequency converting means 5 and outputs a resulting digital signal.
A digital quadrature detecting means 7 performs digital quadrature detection on the real signal received from the A/D converter 6 so that the frequency of each carrier coincides with the first IF carrier frequency which is set in the digital quadrature modulating means, and outputs a resulting complex signal.
A carrier leak detecting means 8 detects carrier leak components from the output signal of the digital quadrature detecting means 7 by, for example, a method using the following equations:
                    <                  Formulae          ⁢                                                            ⁢                                                          ⁢          2                >                                                                      CLDetI          =                                    Σ              ⁢                                                          ⁢                              AcprxI                ⁡                                  (                  t                  )                                                      N                          ⁢                                  ⁢                  CLDetQ          =                                    Σ              ⁢                                                          ⁢                              AcprxQ                ⁡                                  (                  t                  )                                                      N                                              Formula        ⁢                                  ⁢        e        ⁢                                  ⁢        2            where
AcprxI(t): I-phase signal of the output of the digital quadrature detecting means;
AcprxQ(t): Q-phase signal of the output of the digital quadrature detecting means;
CLdetI: I-phase carrier leak detection value;
CLdetQ: Q-phase carrier leak detection value; and
N: Number of cumulative addition samples.
A carrier leak compensation value control means 9 determines carrier leak compensation values to be set newly in the carrier leak compensating means 2 from the carrier leak detection values detected by the carrier leak detecting means 8, and sets them in the carrier leak compensating means 2. More specifically, the carrier leak compensation value control means 9 multiplies each carrier leak detection value CLDet by a proper positive coefficient that is smaller than “1” and subtracts a resulting value from the carrier leak compensation value CLCancel.
The frequency converting means 5, the A/D converter 6, and the digital quadrature detecting means 7 are together called a feedback means. The first IF center frequency is arbitrary and may be equal to the baseband frequency or 0 Hz.
FIG. 2 shows the internal configuration of the digital modulating means 1.
Bandwidth limitation filters 111-114 perform bandwidth limitation on input baseband signals which are complex signals (also called an analytical signals) each consisting of an I-phase (in-phase) component and a Q-phase (quadrature-phase) component so that their frequency ranges fall within a bandwidth assigned to one carrier.
Up-sampling means 121-124 up-sample bandwidth-limited, chip-rate transmission signals to a desired sampling frequency. The bandwidth limitation filters 111-114 and the up-sampling means 121-124 may be combined stepwise as appropriate.
Digital quadrature modulating means 131-134 perform digital quadrature modulation (complex frequency modulation) by complex-multiplying the up-sampled transmission signals by local signals corresponding to respective carrier frequencies.
A multicarrier adder 140 outputs, as an output of the digital modulating means 1, a multicarrier signal obtained by additively combining carrier signals produced through the digital quadrature modulation. The multicarrier signal is also a complex signal.
Another technique relating to the invention which is different from the above-described techniques is known in which a linear distortion is estimated from behavior of the output level of an analog quadrature modulator receiving a test signal and a linear distortion including a DC offset is compensated for by using affine transformation (refer to Non-patent document 1, for example).
Patent document 1: JP-A-2004-221653
Patent document 2: JP-A-2002-208979
Patent document 3: JP-A-2002-164947
Patent document 4: JP-A-2002-7285
Patent document 5: JP-A-2001-339452
Patent document 6: JP-A-11-88454
Patent document 7: JP-A-11-27331
Patent document 8: JP-A-10-136048
Non-patent document 1: Hiroshi Suzuki and other one person, “Affine Transformation Linear Distortion Compensation—Application to Linear Signal Transmission Including Equalization in Mobile Wireless Communications—,” The Transactions of the Institute of Electronics, Information and Communication Engineers (IEICE) B-II, IEICE, January 1992, Vol. J75-B-II, No. 1, pp. 1-9.